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Transformation methods

High Accuracy Transformation - Australian NTv2 Distortion Grid

In order to achieve the best possible spatial accuracy for any type of data, it is recommended that the Australian NTv2 distortion grid be used for all transformations between AGD66 and GDA94.

The absolute accuracy is about 0.1m, however users should be aware that the relative accuracy between points over a small area will be maintained.

All the complex mathematical processing was done during the creation of these grid files, leaving your software to interpolate the required shifts and perform a simple addition to complete the transformation.

Many GIS and surveying software vendors have adopted the NTv2 grid transformation as a standard for their applications.

Refer to the GDA Technical Manual on the ICSM website for more information about the NTv2 grid.

To use this high accuracy transformation you will need to use the National or regional grid file covering your State or area. The files relevant to NSW can be downloaded here:

Grid files for other states can be downloaded from the web sites of the relevent jurisdictions. Some states also have sub grids which may be required for certain areas, and users should check for the existence of sub grids with those jurisdictions.

Overview of Other Transformation Methods

These methods may be used where distortion modelling is impractical or inappropriate.

Block Shift

  • The Block Shift is a simple method of applying an average shift determined using one or more points with coordinates in both datums.
  • Average block shifts have been determined for all 1:250,000 map sheets across Australia, these values can be downloaded from the GDA Technical Manual. The accuracy of these shifts is limited to about 10 metres.

Molodensky's Formula

  • Molodensky's formula uses an average origin shift and the difference in parameters of the two ellipsoids.
  • Formulae can be viewed and a spreadsheet for these calculations downloaded from the GDA Technical Manual. Accuracy is limited to about 5 metres.

7 Parameter Similarity Transformation

  • The 7 parameter similarity transformation is a mathematical process which relates two datums by three origin translations, a rotation about each axis and a change of scale. This is a conformal transformation which preserves the shape of data.
  • The absolute accuracy of transformed data will be in the order of 1 metre, however users should be aware that the relative accuracy between points over a small area will be maintained.
  • The following values are known as the Regional NSW / Victorian parameters and supercede the 'LIC93' transformation values for NSW;

AGD66 to GDA94 (NSW/Victoria)

Parameter Value RMS
X axis Translation (metres) -119.353 ±0.08
Y axis Translation (metres) -48.301 ±0.08
Z axis Translation (metres) 139.484 ±0.08
X axis Rotation (seconds) -0.415 ±0.03
Y axis Rotation (seconds) -0.260 ±0.03
Z axis Rotation (seconds) -0.437 ±0.03
Scale (parts per million) -0.613 ±0.11

Note Two different conventions for rotation definitions are in use in the survey industry. The position vector rotation (Bursa-Wolfe model) commonly used in Europe and the coordinate frame rotation commonly used in Australia and the USA. The only difference being the sign of the rotation parameters.

See the GDA Technical Manual for other regional AGD66/GDA94 similarity transformation parameters or for the national AGD84/GDA94 parameters.

To do a 7 parameter transformation between AGD66 and GDA94 coordinates use our online application. (You will also find this application very useful for conversions between latitude/longitude and easting/northing coordinates in the same datum).

GIS users should also refer to our GIS issues page.

While all effort is made to ensure the accuracy and currency of this information, Spatial Services – Department of Finance, Services and Innovation does not accept responsibility for any actions arising from the use of this data.

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